Optical instruments are generally used for recording properties of defined points in a measurement environment. Known examples of such measurement apparatuses are, for example, geodetic apparatuses for surveying, for example the theodolite or a total station. Such apparatuses have angle and distance measurement functions, which allow direction and distance determination for a selected target. The angle and distance quantities are in this case ascertained in the internal reference system of the apparatus and, for an absolute position determination, may possibly also need to be correlated with an external reference system.
For exact distance determination, there is a sighting unit, usually a telescope, having an optical axis, in particular a sighting axis, in the surveying apparatus. A laser beam is coupled into it in order to measure the distance to measurement points, for example by means of pulse time-of-flight determination. Owing to the high required measurement accuracy in the mm range, most instruments are currently equipped with an electro-optical distance meter, which operates according to the phase measurement principle. These furthermore offer advantages in the case of distance-measuring theodolites owing to their relatively small volume.
Objectives, mirrors, lenses or other optical pieces must be installed and adjusted with high precision in the housing or in the sighting unit, so that no undesired deviations or reflections occur in the optical beam path and maximally accurate values can therefore be determined in a measurement. The input coupling element for the laser beam must also be fitted exactly therein, so that the direction of the beam can as far as possible be aligned, and is kept as far as possible time-invariantly, centrally and parallel to the sighting axis.
The accurate mounting of optical components or optical assemblies in suitable holders can, as is known, be ensured by carrying out elaborate processing of the components. Owing to the narrow tolerances, the components must be adapted as accurately as possible to one another, in order to achieve exact positionability. These processes are usually very cost-intensive and technologically demanding. Even very minor deviations in the processing of the components can lead to insufficiently stable connections and therefore necessitate reprocessing or the replacement of at least one of the components. Furthermore, thermal effects such as differing expansion behavior of the pieces can only be compensated for with difficulty since the components are usually made of different materials. Therefore, a relative position change of the components with respect to one another can occur, and the beam path of the optical piece can be modified in such a way that the required accuracy or even the functionality of the apparatus can no longer be ensured.
It is furthermore known that elastically compressed sealing elements, for example O-rings, are used for this mounting and placed and installed between the components. The tolerances for the manufacture of the components can therefore be increased and possibly necessary reprocessing operations can be obviated. Sometimes, said sealing elements are also used to ensure a degree of play in the radial direction.
A releasable and stress-free holder of an optical component placed in a centered fashion in a frame is known from DE 19 924 849. The optical component is in this case provided with a chamfered surface, inclined in the direction toward the frame in the edge region to be held, and the frame has a grooved recess open in front of the optical component in the axial direction. An annular element which is elastically deformable in the axial direction, for example an O-ring, is pressed in between the chamfered surface and the grooved recess.
The deformability in the axial direction allows tolerance compensation between the chamfered edge and the grooved recess, as well as compensation for material deformation in the axial direction. Minor tolerances between the chamfered edge and the position of the grooved recess are compensated for by an elastically deformable annular element. Material modifications in the axial direction can likewise be absorbed by the annular element. In addition, the annular element constitutes impact and vibration protection.
DE 10 043 344 furthermore discloses that an annular groove is formed on a circumferential surface of a lens, and the lens is radially and axially held exclusively by a connection which is formed by elastic elements on the lens frame, with free ends which engage radially into the annular groove. The different thermal expansions of the lens frame and the lens are in this case compensated for in the radial direction by means of the spring action of the segments. Owing to the fact that the lens is connected to the frame exclusively by means of elastic segments, dynamic loads are only transmitted to the lens in an attenuated fashion.
A disadvantage of using elastic sealing elements for the mounting is the high friction and the concomitant “stick-slip effects”, which can occur during mounting and alignment and therefore make adjustment of the components more difficult. Distortions may in this case occur in the material and cause an asymmetric force distribution, which can lead by thermal or shock influences to relative displacements of the components. Furthermore, owing to incompressibility, absence of plasticity and relatively wide tolerance quality, these elements have high contact forces so that these parts is not optimally suitable for such mounting purposes. High forces can therefore occur between the components, which may in turn give rise to unstable behavior, and vibrations which occur cannot be attenuated in the necessary manner.